It is known to use the wave field emanating from a person in an analogous way as a detection field.
For example, such an electronic surveillance system based on the infrared radiation emanating from persons is described in DE 38 32 428 A1.
Finally, the use of microwave radiation for movement detection on open land or inside premises which is based on the modulation of a microwave beam by objects moving in the area exposed to the radiation is also known.
Furthermore, also known are search devices which operate with microwaves and are used for locating persons cut off from the outside world by earthquakes or avalanches.
The use of microwaves for locating buried persons is based on the fact that microwaves are to some extent capable of penetrating rubble and rock debris and that the reflected components exhibit different characteristics, depending on the material.
K. Chen et al. "An X-band microwave life-detection system", IEEE Transactions on biomedical engineering, Vol. BME 33, No. 7, July 1986 discloses such an electronic life detector, operating with microwaves, as mentioned above.
In the case of this electronic life detector, microwaves of a certain frequency are continuously radiated into a spatial area to be investigated.
If there is a living person there, the signal reflected from the scanned area has an unmodulated component and a component modulated by the body functions, specifically breathing and heartbeat, of the person cut off from the outside world. The unmodulated component of the reflected signal is eliminated by a compensation loop which can be controlled in its phase and attenuation constant.
The informational content of the modulated component of the reflected signal is thus selected by a phase comparison with the emitted (scanning) signal.
In the cited publication there is also described a modification of the process suitable for contactless patient monitoring which makes do without a directional aerial and with an output power of 0.1 mW.
This known apparatus is, however, suitable only to a limited extent for use under unfavourable conditions in which the received reflection signals are very weak and/or fluctuating greatly in their intensity, and in particular for a preventive surveillance of buildings.
Furthermore, U.S. Pat. No. 4,967,751 discloses an apparatus in which the microwaves radiating through a living body are subjected to a frequency analysis. This process is not suitable, however, for the detection of living beings, since a building or an area where people are buried is not usually accessible from opposing sides.